Today I noticed two of my dwarf rainbows had cotton mouth. I'm pretty sure they had it at the store and I just didn't catch it... I'm a bit annoyed because it's an LFS that's a bit of a drive from me and the fish there are much pricier than some other places. For what I pay there, I expect healthy fish. But there's not much I can do about that now... Anyway, I'm looking for advice on how to go about dealing with it.

The tank they were in is a 30 gallon breeder. I had a school of 6 dwarf rainbows fish, a school of 6 zebra danios, and a pair of albino kribs. It's filtered, heated, has an air stone, etc. and I've started adding live plants as well. The substrate's sand, most of the decorations are rocks, with a few plastic plants and some caves in addition to the live plants I just added recently.

When I noticed the fungus this morning, I moved the betta I had in a 10 gallon into a 2.5 gallon so I could use his 10 gallon tank to isolate the sick fish. I emptied the 10 gallon and moved it next to the 30 gallon. I siphoned out water from the 30 gallon into the 10 gallon, transferred the sick fish to the 10 gallon, and then put 10 gallons of new water into the 30 gallon. So now the sick fish are isolated.

As of right now, I don't have an anti-fungal medicine on hand. I plan to go and buy some as soon as I get a chance. But, my question is do I have to medicate both tanks? None of the other fish in the 30 gallon are showing any symptoms of illness, but do I need to medicate just to be safe? If I do, can I leave the decorations and plants in while medicating or should I take everything out of the tank? Also, what meds should I get to treat the fish with?